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Hi: I think this topic was covered before, but I can't find it in the old threads. So if you can bear with me. My heat pump seems to short cycle ontly at night when the heat load is low. Recently, I had the heat pump serviced for another matter that I have discussed before so I won't go into it again, but the tech said that the filter that I am useing is pretty restrictive and that I should think about getting another one. He said that when the fan coil was running (Carrier FV4A) he pulled the filter out that I am useing the CFM's really slowed down, indicating the filter is too restrictive. Would some of you guy's let me know what is the best filter to use that will be the least restrictive, yet will keep my evaporator clean.

Filters ..

Return Air flow will likely be adequate with filter area based on 300 CFM/ square foot ( ~ 2 CFM / square inch).

Return Air DUCT should be sized at about 600 FPM.

For example, 4 ton unit air flow is generally 1,600 CFM.
Total filter area should be 5.5 square feet (~ 800 square inches). Perhaps, one 18" x18" and three 12" x12" (or 14"x14") filters would be appropriate for 3 bedroom home which is less than 2,200 square feet.

18" x 18" filter grille can handle up to 800 CFM with a short run of 14" [ 749 FPM ] or 25 + foot run of 16" duct [ 573 FPM ].

USE a Deep (2 to 5") pleated MERV 7 that is adequately sized or oversized 1" deep pleated.

Designer Dan
It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE
Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

What does the word good actually mean?

Thorton;

The best news is at least you're aware that there is a filter in the system <grin>. That said, words like best and worst are meaningless. If you are truly concerned you need to determine the pressure drop (IWC) across the filter verses the total pressure available within the duct system. Careful evaluation of the required value range given by Carrier should yield the answer you seek. Want some interesting reading? Order a copy of The ACCA Manual D (ISBN 1-892765-00-4) through your inter-Library loan system. BTW: If you're an HVAC tech and haven't read this manual, do so at your earliest convenience (honest counsel).

Note: Measurements with the filter in place and then removed are meaningful (does this make sense?). The vendor of the air-handler (Carrier in this case) designed their system to operate within a performance envelope (high verses low total pressure). There are restrictions on Velocities that have to be considered (residential velocities are generally limited to below 900FPM). The entire air distribution system must be viewed as a dynamic equation in which changes to any single component can have significant up & downstream consequences. Does this make sense?

Please remember that there is a significant difference between clean verses a dirty filter. It is the later that causes so many problems in a marginal system.

Ask yourself what am I trying to do? Is it protecting the equipment, or does Indoor Air Quality dominate your priorities. Yes this is science and there are compelling reasons to take the effort to do it correctly.

Any individual making recommendation without the benefit of airflow measurements mentioned above isn’t a HVAC professional but rather an opinionated soul with some other agenda, enough said.

Hope this helps more than bewilders… There simply aren’t black and while answers without the appropriate measurements and equipment operating parameters.